Crystal force and pressure transducers

ABSTRACT

A force or fluid pressure transducer comprises a plate-like crystal. The force, to be measured, is applied by two seatings disposed on opposite edges of the crystal. The transducer can measure pressure when a diaphragm is added to provide the force. 
     Two portions of the crystal are maintained in continuous oscillation by feed-back circuits. These portions change their frequencies by different amounts when the force is changed. The difference between the two frequencies is a very accurate measure of the force and forms the output of the transducer. 
     Structures to secure the crystal have, in the past, proved difficult to manufacture. The present invention describes a securing structure for the crystal which substantially improves and facilitates securing and housing the crystal to provide high accuracy under all normal working conditions of the transducer.

BACKGROUND OF THE INVENTION

This invention relates to oscillating crystal transducer systems of thetype described in my U.S. Pat. Nos. 4,703,216, 4,485,323, 4,439,705,4,175,243, 4,126,801, 4,067,241, 4,020,488, and 3,891,870.

According to these patents a force or fluid pressure transducer containsone or more plate-like oscillating crystals which sit up-right onseatings disposed on a base member which forms part of the transducerhousing. Means are provided for transmitting a force to the crystalalong the direction of its surface in order to produce, in response tosaid force a change of the resonant frequencies of the crystal.

The force may originate from a fluid pressure where it is required tomeasure this pressure being applied to a diaphragm which forms part ofthe transducer enclosure. The center-point of this diaphragm is arrangedto apply the force to the crystal.

In the later patents the crystal has on it two oscillating portions, oneof which is substantially un-responsive to the force. This arrangementis advantageous because the change in difference frequency between thetwo portions is a very accurate measure of the force.

Although in principle, the existing patents disclosed transducers whichperformed with increasing accuracy over a progressively wider range ofconditions as development proceeded, difficulty in manufacturing theunits in a satisfactory manner and thereafter providing adequatehermetic sealing has continued to exists. The present invention providesmeans to overcome these problems.

The earlier problems were such as to make the transducers unduly bulkyand as a result the masses of the components could result in excessivesensitivity to shock and vibration.

Further, although the required position of the crystal between itsseatings was known, it was difficult to adjust the crystal into thisposition by this means to reduce inaccuracies resulting from ambienttemperature changes.

SUMMARY OF THE INVENTION

The invention comprises a quartz crystal having on it pairs ofelectrodes disposed on corresponding regions on opposite sides of thecrystal. The crystal is maintained in continuous oscillation by afeedback circuit or circuits connected to said electrodes.

The change in the frequency of the oscillation originating from thecrystal is responsive to the force to be measured. This force is appliedby seatings disposed at the periphery of the crystal and directed alongthe plate surface.

The present invention resides in a U-shaped rigid clip inside which thecrystal is secured by a plate-like spring which is essentially flat.This is fitted across the mouth of the clip. The securing permitsaccurate adjustment of the crystal position between seatings beforeinsertion of the clip into the hermetically sealed housing. Theadjustment is essential to attain high accuracy of measurement by thetransducer particularly under conditions of variation of ambienttemperature.

Heretofore, without this design of clip, (for exmple in the transducershown in FIG. 4 of U.S. Pat. No. 4,485,323) it was excessively difficultto adjust the crystal in position and still maintain adequate accuracyof the transducer and at the same time provide hermetic sealing of thedevice.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 show prior art ane included to facilitate explanation.These are from my U.S. Pat. Nos. 4,175,243, and 4,480,323 respectively.

FIGS. 3 and 4 show plan and elevation views respectively of oneconfiguration of the transducer incorporating the present invention.

FIG. 5 shows a cross-sectional view of the transducer of FIG. 3.

FIG. 6 shows a cross-sectional view of the transducer with overloadrelieafs and added features.

DESCRIPTION OF PRIOR ART

Referring firstly to the prior art in FIG. 1. (U.S. Pat. No. 4,175,243)a circular plate-like crystal 1 has on its surface two pairs ofelectrodes 2a and 2b, and 3a and 3b disposed on corresponding opposedareas on each side of the plate. Connection tabs 4 and 5 on the front ofplate 1 and common connection tab 6 on the rear side enable feedbackamplifiers to be connected to the electrodes to maintain them incontinuous oscillation in the manner well-known in the communicationindustry. Force F applied to disc 7 in the center of disphragm 8 iscarefully directed via pin 9 to spigot 10. This spigot is secured by twoflat spring cantilevers 11 and 12. Only the vertical component of ForceF is thus transmitted to the upper seating 13 which applies compressionto crystal 1 along the direction of its surface. Lower seating 14 sitsbelow crystal 1. Bolts 15 secure the housing rings to form a frame whichsits inside a cannister 16. Said cannister together with lid 18 forms ahermetically sealed enclosure. Change of force F results in a differencefrequency change between electrode pairs 2a, 2b and 3a, 3b. Thisdifference frequency forms the transducer output.

FIG. 2 shows a prior art transducer (U.S. Pat. No. 4,485,323) that isfurther improved. The improvements reside in reduction of crystal sizeby cutting away all of the crystal plate 1 except for the relevant partcontaining the electrode pairs. Higher stress level between the primaryforce-responsive electrode pair 17 results in substantially largerfrequency change and higher accuracy of the transducer, as does alsooscillation in overtone modes.

Further, as shown in FIG. 2, the second electrode pair 18 is arranged tohave additional seatings 19 and 20 which permit adjustment to compensatefor temperature change. However, later methods of achieving thiscompensation have proved more effective and these will be described inthe text which follows.

DESCRIPTION OF THE PREPARED EMBODIMENTS

Referring to FIGS. 3, 4 and 5 which are respectively plan and elevationviews of the transducer of said FIG. 3., the new deveopment relies uponthe following innovations:

Crystal 21 is mounted within a U-shaped frame 22. Said frame holds lowerseating 23 for crystal 21 inside its base. U-shaped frame 22 is bridgedat its mouth by flat spring member 24. The latter is secured at one endby projections 25 and 26 which are cut out from the top edge of saidframe. The other end of spring 24 is secured by adjusting screw 27 shownin FIG. 5. This screw passes through a hole in said spring.

Upper seating 28 is secured to flat spring 24 and adjusting screw 27 isarranged to preload said seating 28 against the upper edge of crystal21. Thus crystal 21 is firmly clamped between the seatings 23 and 28.

Frame 22 is secured by screws 29 and 30 which pass through anti-shocktabs 29a and 30a. These tabs are parts of said frame and thus secure theframe in a hermetically sealed enclosure 33 completed by lid 34. Bothparts 33 and 34 have annular diaphragms 35 and 36 formed by plates 37and 38 at the center of each. Said plates are joined by rod 39.Projection 40 from plate 37 enables force F to be transmitted to crystal21 via spring 24 and seating 28.

Thus, if force F originates from a fluid pressure P applied to diaphragm35 the change in the frequency of crystal 21 will be a direct measure ofpressure P.

Further, if force F originates from the difference between two fluidpressures P₁ on diaphragm 35 and P₂ on diaphragm 36 then the frequencychange of said crystal will be a measure of the difference between P₁and P₂.

The hermetically sealed enclosure formed by housing 33 and lid 34 iseither filled with a dry inert gas to obviate contamination of thecrystal or in some applications the enclosure may be evacuated toprovide very high accuracy units.

The transducer may also be used as a load cell to measure force F.Because pressure P₁ will equal P₂ the unit is compensated for ambientair-pressure changes.

Yet further, the differential pressure transducer can be employed forfluid flow rate measurement when it is used to sense pressure dropacross an orifice.

Reference is now made to FIG. 6 which shows the transducer of FIGS. 3, 4and 5 but with the added features of overload reliefs which operate inthe event of excess force above designed full range when applied at theupper or lower diaphragms.

The reliefs provide protection for the crystal particularly in the eventof excessive direct load, shock or vibration to the transducer.

Protection of two kinds is afforded. Firstly protection is providedagainst catastrophic damage to the crystal and secondly prevention ofsignificant shift of normal zero or fixed load output of the transducer.For example, double overload of the unit would not normally causeinferior operating of the transducer. However, the overload reliefsprovide operation up to several times full rated load of the transducer.

In FIG. 6 protection is achieved by locating a stainless steel ball 41in recess 42 within plate 43.

In normal operation ball 41 is held in the position shown, by coilspring 44. However, during large over-pressure on the top of diaphragm45 spring 44 is compressed by said ball and over-pressure can increaseuntil a projection 46 on rod 47 closes the gap 48 with the lower part offrame 49 whereupon no further force is introduced to crystal 50.

Yet further, if excessive pressure is applied to lower diaphragm 51 thengap 52 will be closed when stop 53 touches the underside of frame 30thus avoiding undesirable excessive travel of said two diaphragms whichwould otherwise become deformed.

Another protective feature of the transducer consists in provision of ananti-shock mounting for frame 52 of FIG. 6. This is achieved by fittinga sheet of plastic 55 between frame 52 and enclosure 56, at the timesaid frame is fitted into said enclosure. The plastic provides aresilient and damped separating medium between the two parts and whenthe plastic having suitable characteristics is employed enables thetransducer to withstand substantial shock and vibration withoutsustaining damage.

Tendency for crystal 50 to swivel in its seatings under shock conditionsis counteracted by the careful selection of the length and thickness ofthe wires 57, connecting the crystal electrodes 58 to the maintainingcircuit.

Reference to U.S. Pat. No. 4,439,705 shows that the mass loadingfeatures claimed therein can equally well be applied to the crystal usedin this present invention to compensate for zero shift; i.e., undesiredfrequency shifts arising from the effect of ambient temperature changein the transducer.

Reference to U.S. Pat. No. 4,020,448 wherein oscillation of two crystalsis claimed, shows that this principal could also be applied withadvantage to additionally reduce the zero shift of the output signal inthe current invention.

A further feature of the transducer is its alternative use as a linearaccelerometer. The dashed line 54 in FIG. 6 shows the location of a masswhich may be secured to rod 47.

With this mass the transducer becomes sensitive to acceleration alongaxis 59.

SUMMARY OF ADVANTAGES OVER EARLIER PATENTS

1. The crystal can be assembled into a frame and adjusted in positionprior to insertion of said frame into the main housing which issubsequently hermetically sealed. This is a major improvement over myU.S. Pat. Nos. 4,175,243 and 4,480,323 wherein assembly of the crystalproved difficult.

2. The crystal is more firmly secured than in former Patents.

3. The overall assembly is more compact than in previous Patents.

4. The new assembly forms a differential pressure transducer in additionto its ability to be arranged for absolute pressure measurement as wasformerly the case.

5. The new transducer is an improvement over the looped spring member ofU.S. Pat. No. 4,940,915 which suffered inaccuracy from shock andvibration effects because the mass of said spring 24 arising from itstotal unsupported length including seating member 23 is significantlygreater than that of the short flat spring bridging the U-shaped frameof the new invention described herein.

I claim:
 1. A force and pressure transducer comprising:a plate-shapedpiezo-electric crystal having a peripheral edge, a first pair ofelectrodes disposed on a corresponding region on opposite sides of afirst portion of the crystal, a second pair of electrodes disposed oncorresponding regions on opposite sides of a second portion of thecrystal, means for energising the electrode pairs to maintainoscillation of the first and second crystal portions at differencefrequencies, a U-shaped frame containing inside the bottom of itsU-shape a first seating member engaging the peripheral edge of thecrystal, a substantially flat leaf spring bridging the gap at the openend of the U-shaped frame and having on it a second seating memberengaging the peripheral edge of the crystal and arranged to apply aforce along a line extending through said first oscillating crystalportion to said first seating member.
 2. A force and pressure transduceraccording to claim 1, including means for computing the differencefrequency between the two oscillating portions and making thisdifference frequency available as the transducer output.
 3. A force andpressure transducer according to claim 1 the pair of seating membersbeing arranged in positions along the crystal periphery to cause theforce sensitivity of the transducer output to be independent oftemperature changes.
 4. A force and pressure transducer according toclaim 1, in which said first and second crystal portions are arranged tooscillate in an overtone mode.
 5. A force and pressure transduceraccording to claim 1 in which said first crystal portion or said secondcrystal portion is arranged to oscillate in an overtone mode.
 6. A forceand pressure transducer according to claim 1 in which said first andsecond crystal portions are arranged to oscillate in different overtonemodes.
 7. A force and pressure transducer according to claim 1 in whichthe crystal is surrounded by a hermetically sealed enclosure.
 8. A forceand pressure transducer according to claim 1 in which said enclosure isfilled with an inert gas.
 9. A force and pressure transducer accordingto claim 1 in which said enclosure provides a vacuum in which thecrystal resonates.
 10. A force and pressure transducer according toclaim 1 which is arranged to measure a single force.
 11. A force andpressure according transducer to claim 1 which is arranged to measurethe difference between two forces.
 12. A force and pressure transduceraccording to claim 1 which is arranged to measure a single pressure. 13.A force and pressure transducer according to claim 1 which is arrangedto measure the difference between two pressures.
 14. A force andpressure transducer according to claim 1 in incorporating over-loadfeatures.
 15. A force and pressure transducer according to claim 1 inwhich the force originates from a mass secured in such a manner as toapply the force due to acceleration of said mass to form anaccelerometer.
 16. A force and pressure transducer according to claim 1in which undesired variation in frequency of the output signal withtemperature is compensated by mass loading of the crystal plate.
 17. Aforce and pressure transducer according to claim 1 in which an adjustingscrew is provided to influence the magnitude of the force exerted by thespring upon the crystal.
 18. A force and pressure transducer to claim 1in which the frequency variation with temperature of said oscillatingportions is employed to compensate the transducer output for the effectof temperature variation of the transducer.
 19. A force and pressuretransducer according to claim 1 in which the electrical connections tothe resonators are additionally employed to secure the crystal in itsposition under shock and vibration conditions.
 20. A force and pressuretransducer according to claim 1 in which said U-shaped frame issupported within said hermetically sealed enclosure by damping materialin such a manner as to provide shock resistance of the completedtransducer.